Figure 1. Sample locations and local annual photoperiod-temperature ellipsoids. (A) Distribution map of localities of common vole samples used in this study. Location numbers with corresponding environmental proxies are listed in Table S1. (B) Annual patterns (counter-clockwise) of photoperiod and 10-year average monthly ambient temperature for each sample location in Western Europe and (C) in Eastern Europe. Temperature data were obtained from the closest weather station (always within 110 km of sample location) obtained from Wunderground (https://www.wunderground.com/). Photoperiod was obtained from https://www.timeanddate.com, and is based on civil twilight times at dawn and dusk, which is the timing at which log light intensities change most rapidly (Daan & Aschoff, 1975; Hut et al., 2013). Civil twilight incorporates geographical and seasonal variation in the duration of twilight (Nielsen, 1961), and is therefore considered as the moment of ‘lights on’ and ‘lights off’ for biological systems (Hut et al., 2013). Dotted lines indicate a temperature threshold at 6.6°C from which the corresponding predicted critical photoperiod (pCPP) in spring (ellipse crossing the 6.6 °C line for the second time) is used as a proxy for onset of grass growth and consequently as a proxy for onset of the favorable season. Regional pCPP’s are listed in Table S1. Colors indicate latitude, ranging from 42°N (yellow) to 59°N (purple).